Field of the Invention
The present invention relates to the supply of hydrogen from a metal hydride storage system.
Related Art
Current technologies for storage and transport of hydrogen include compressed gas (in steel and composite cylinders and tubes) and liquid. Storage in metal hydrides is attractive when a low operating pressure and restricting the amount of gaseous hydrogen are desired. Metal hydrides that have been used in commercial applications are of the bcc type with a gravimetric capacity below 2% and a high cost which prevents their use for bulk storage applications, considering that this gravimetric capacity is reduced by the pressure vessel required to contain the hydride.
Magnesium hydride is a potentially attractive material for bulk storage with a capacity as high as 7.6 wt %. Magnesium hydride has however suffered from difficult activation and poor kinetics. New synthesis methods have been developed to solve these issues, but they are sophisticated and not industrialized. AU 2005313837 discloses a new alloy of magnesium and nickel having good capacity and which is prepared by casting, thereby rendering it relatively inexpensive and suitable for bulk storage applications.
As with other hydrides, it is possible to use hydrogen storage systems based on this magnesium hydride alloy to deliver hydrogen of higher quality at the point of use than the hydrogen that is used for filling. Since the metal hydride only absorbs hydrogen reversibly, the non-absorbed gas will be concentrated as contaminants in a non-adsorbed form—the residual gas present in the free volume of the hydride bed. When demand for hydrogen from the point of use is initiated and hydrogen is allowed to desorb from the magnesium alloy hydride, the initial flow of hydrogen will be a mixture of desorbed hydrogen and the contaminant-rich gaseous phase. This initial flow of contaminant-rich hydrogen only represents a relatively small amount of the overall gas flow from the storage system throughout the entirety of desorption from the magnesium alloy hydride. Because it represents only a small fraction of the overall hydrogen desorbed from the storage system, if this initial flow exceeds the specifications for contaminant levels required by the point of use, the gaseous hydrogen rich in contaminants is typically discarded (e.g., vented or burned). The venting or burning of a portion of the hydrogen is timed to assure that the hydrogen delivered to the hydrogen user has the required quality with no contaminates. While being a small portion of the total hydrogen desorbed, venting or burning this hydrogen does represent a loss of valuable product.
Two other difficulties remain in using Mg-based hydride systems to deliver hydrogen. One is decrepitation and the resulting formation of loose hydride particles that can be entrained with the flow of hydrogen to the consuming application. The second issue is methane that can result from a reaction of hydrogen with carbon present in steel making up the pressure vessel. The carbon may be incorporated in the steel alloy or a surface contamination resulting from the forming process.
Several hydrogen purification and/or storage solutions have been proposed.
JP 59047599 A2 discloses a storage vessel in which a Ti—Mn hydride (which operates near ambient temperature) is mixed with an adsorbent such as activated carbon. The purpose of the adsorbent is to remove impurities from the hydrogen stream. This proposal does not solve the above-noted problem for the following reason. In comparison to hydride systems not including an adsorbent mixed with the hydride, the storage vessel of JP 59047599 A2 will have a relatively lower gravimetric capacity due to the necessity of mixing the Ti—Mn hydride with the adsorbent. This problem is exacerbated if the adsorbent is also counted upon to remove any methane that is generated within the storage system.
U.S. Pat. No. 7,736,609 discloses a hydrogen purification system in which CO and CO2 are converted to methane by a catalytic reaction. Methane is removed by venting prior to desorption from a hydride material. However, it is evident that some methane will remain in the purified stream at a concentration around hundreds of ppm.
U.S. Pat. No. 6,508,866 discloses a purification system including a mixture of water-absorbing material, metal hydride, and noble metal particles that is intended for removal of water vapor, and other contaminants such as CO, CO2, and O2. This solution does not address the issue of impurities formed by operation of the metal hydride system.
EP0315582 discloses purification by a getter material upstream of purification in a metal hydride purification system. Again this does not address impurities formed by operation of the metal hydride system.
Particle entrainment may be prevented by using a thermoplastic binder, such as disclosed by U.S. Pat. No. 8,691,472. However, this type of solution is not applicable to high-temperature metal hydride systems since the thermoplastic material would flow and not withstand the operating conditions.
U.S. Pat. No. 7,504,083 discloses the use of silica-based gel in the metal hydride storage system. While silica gel should resist high temperature, this addition of silica gel adds weight and reduces the volume available for storage of hydrogen.
KR 100837973 A discloses a powder filter, in the form of a supported mesh, for preventing downstream contamination by particles. The drawbacks of this type of filter are that fine particles can plug the filter, creating a detrimental pressure drop. Any abrupt change in flow can dislodge a portion of the collected particles on the filter and leading to a contamination of the gas stream with entrained particulates.
CN 202048351 discloses a filter sheet similar to the filter of KR 100837973 with similar drawbacks.